The invention relates to a high-pressure metal halide discharge lamp with a power rating of at most 100 W, provided with a discharge vessel having a translucent ceramic wall with a thickness d, which discharge vessel encloses a discharge space in which two electrodes, each provided with an electrode tip, are arranged with the electrode tips at a mutual distance EA, which discharge vessel contains an ionizable filling including a halide of sodium, and which discharge vessel is cylindrical over the distance EA and has an internal cross-sectional diameter Di.
A lamp of the kind mentioned in the opening paragraph is known from EP 215524. The known lamp, which has a power rating of 40 W, has a discharge vessel wall thickness of 0.45 mm. The ionizable filling of the discharge vessel comprises besides Hg also halides of Na, Tl, and In. The lamp has good color properties, in particular a good color point with coordinates (x;y), and good values both for the general color rendering index Ra and for the color rendering index R9 designating the rendering of red. This renders the lamp basically highly suitable for interior lighting applications. In this lamp that a good color rendering is possible when Na halide is used as a filling ingredient of a lamp, and the Na pressure is so high during operation that a strong widening and inversion of the Na emission in the Na-D lines takes place. Since the Na is present in excess quantity, this requires a high temperature of the coldest spot Tcs in the lamp vessel of, for example, 1000 K (730xc2x0 C.) during lamp operation. The Na-D lines assume the shape of an emission band in the spectrum with two maxima having a mutual interspacing xcex94xcex in the case of inversion and widening of these lines. The requirement that Tcs should have a high value excludes under practical conditions the use of quartz or quartz glass for the discharge vessel wall and necessitates the use of a ceramic material.
The term xe2x80x9cceramic wallxe2x80x9d in the present description and claims is understood to mean both a gastight wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al2O3, and a wall made of metal nitride, for example AlN.
A disadvantage of the known lamp is that the lamp has a comparatively short life in practice owing to attacks on and cracking of the discharge vessel.
The invention has for its object to provide a measure for realizing a lamp having a longer useful life. For this purpose the thickness d of the wall is at least 1.2 mm.
The use of a comparatively thick wall advantageously leads not only to a better heat transport from the portion of the wall between the electrodes to the comparatively cool ends of the discharge vessel, but most of all to an increase in heat radiation emitted by the wall of the discharge vessel. Compared with a wall according to the prior art, the thick wall here leads to a lower wall temperature as well as to a smaller temperature gradient across the wall. The latter has a particularly favorable influence on a reduction of chemical processes in which the transport of components plays a major role. It is true that the thicker wall in itself leads to a reduced attack and a smaller risk of fractures, but on the other hand it results in a reduction of the temperature of the coldest spot Tcs, all other parameters remaining the same. It is found in the known lamp that the color properties, in particular the color point and the general color rendering index, are highly sensitive to changes in Tcs.
A reduction in this sensitivity to changes in Tcs is achieved to a high degree when the ionizable filling is free from In. A further improvement can be achieved in that the ionizable filling comprises a rare earth halide. A strongly improved color stability throughout lamp life is also realized thereby. Dy was found to be a particularly suitable ingredient for the ionizable filling in this respect.
Preferably, the relation 0.4xe2x89xa6EA/Dixe2x89xa61.5 is complied with in a lamp according to the invention. The advantage of this is that, in spite of the thick wall, the Tcs value lies in a range between 1200 K and 1300 K, while at the same time the maximum temperature of the discharge vessel wall remains limited to 1400 K. It was found in experiments that a value of xcex94xcex between 12 nm and 60 nm can be realized for a value of Tcs in the range from 1200 K to 1300 K. To realize a lamp radiating white light with a general color rendering index of at least 90, it is desirable for the value of xcex94xcex to lie between 12 nm and 60 nm.
It is found for a ratio EA/Dixe2x89xa60.4 that a considerable blackening of the discharge vessel wall occurs in a comparatively short time owing to convection flows in the lamp vessel. Such a blackening is disastrous for good color properties of the lamp. If the ratio is chosen to be greater than 1.5, on the other hand, it is found in practice that a value of the general color rendering index greater than 90 cannot be combined with a long lamp life without unacceptable loss of luminous efficacy.